Humans are naturally polygamous The history of western civilization aside, humans are naturally polygamous. Polyandry (a marriage of one woman to many men) is very rare, but polygyny (the marriage of one man to many women) is widely practiced in human societies, even though Judeo-Christian traditions hold that monogamy is the only natural form of marriage. We know that humans have been polygynous throughout most of history because men are taller than women.

Among primate and nonprimate species, the degree of polygyny highly correlates with the degree to which males of a species are larger than females. The more polygynous the species, the greater the size disparity between the sexes. Typically, human males are 10 percent taller and 20 percent heavier than females. This suggests that, throughout history, humans have been mildly polygynous.

Relative to monogamy, polygyny creates greater fitness variance (the distance between the “winners” and the “losers” in the reproductive game) among males than among females because it allows a few males to monopolize all the females in the group. The greater fitness variance among males creates greater pressure for men to compete with each other for mates. Only big and tall males can win mating opportunities. Among pair-bonding species like humans, in which males and females stay together to raise their children, females also prefer to mate with big and tall males because they can provide better physical protection against predators and other males.

In societies where rich men are much richer than poor men, women (and their children) are better off sharing the few wealthy men; one-half, one-quarter, or even one-tenth of a wealthy man is still better than an entire poor man. As George Bernard Shaw puts it, “The maternal instinct leads a woman to prefer a tenth share in a first-rate man to the exclusive possession of a third-rate one.” Despite the fact that humans are naturally polygynous, most industrial societies are monogamous because men tend to be more or less equal in their resources compared with their ancestors in medieval times. (Inequality tends to increase as society advances in complexity from hunter-gatherer to advanced agrarian societies. Industrialization tends to decrease the level of inequality.)

Most women benefit from polygyny, while most men benefit from monogamyWhen there is resource inequality among men—the case in every human society—most women benefit from polygyny: women can share a wealthy man. Under monogamy, they are stuck with marrying a poorer man.The only exceptions are extremely desirable women. Under monogamy, they can monopolize the wealthiest men; under polygyny, they must share the men with other, less desirable women. However, the situation is exactly opposite for men. Monogamy guarantees that every man can find a wife. True, less desirable men can marry only less desirable women, but that’s much better than not marrying anyone at all.

Men in monogamous societies imagine they would be better off under polygyny. What they don’t realize is that, for most men who are not extremely desirable, polygyny means no wife at all, or, if they are lucky, a wife who is much less desirable than one they could get under monogamy.